X-ray fluorescence (XRF) analysis is a good way of detecting the presence of heavy metals in samples made of e.g. plastic or other matrix material. For example, enforcing the RoHS (Restriction of Hazardous Substances) directive of the European Union requires capability of measuring, how much a sample contains lead, cadmium, mercury, hexavalent chromium, and polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants, to which purpose XRF is well suited. The two last-mentioned substances are conveniently detected by measuring the amount of bromine in the sample. Other standards exist also that require good XRF detection capabilities.
Detecting heavy elements in an XRF measurement requires the incident X-ray radiation that is used as excitation to be relatively energetic. The natural output spectrum of a typical 40 kV X-ray tube does not have the best possible shape, for a number of reasons. As an exemplary material to be detected we will consider cadmium (Cd). If the anode material of the X-ray tube is rhodium (Rh), only the continuous high-energy bremsstrahlung part of the incident radiation is useful, because the K-lines of Rh (20.2 keV and 22.7 keV) are below the K-shell absorption edge (26.7 keV) of Cd. Some line structure in the excitation spectrum would be needed to perform the so-called matrix correction, which means deconvolving the detected fluorescence spectrum into Compton scattered and coherently scattered parts and analysing their relative intensities. If the anode material is heavier, like tungsten (W), the proportional intensity of the useful excitation radiation is higher and the characteristic L-line peaks of W would basically be available for matrix correction. However, with a W anode the Compton scatter peaks are relatively close to the coherent scatter peaks, which makes spectrum deconvolution difficult. Also, the L-lines of W (8.39 keV and 9.67 keV) overlap with spectral lines of some interesting materials to be detected, which means that they must be filtered out, again leaving the excitation spectrum without the required structure.
Other known anode materials for so-called high-Z X-ray tubes include but are not limited to rhenium (Re), platinum (Pt), and gold (Au).
It is known from prior art to shape the spectrum of incident X-ray radiation by using a filter between the X-ray source and the sample. A molybdenum (Mo) filter in front of a 40 kV X-ray tube with a tungsten anode would serve many purposes. It would effectively filter out the L-line peaks of W, and give rise to characteristic incident radiation peaks of Mo at 17.5 keV (K-alpha), 19.6 keV (K-beta), 2.29 keV (L-alpha), and 2.39 keV (L-beta). The energies are high enough and far enough apart for good separation of Compton and coherent scattering peaks, and do not overlap with those spectral lines that are important for analysing the substances mentioned in the RoHS directive. However, such a Mo filter also absorbs significant quantities of the bremsstrahlung above 26.7 keV, which would be needed for Cd excitation. The thickness of the Mo filter is an awkward tradeoff: the thinner the filter, the less there occurs unwanted absorption at energies above 26.7 keV, but the lower is also the intensity of the characteristic Mo peaks, which would be needed for the matrix correction, and the weaker is the desired effect of filtering out the unwanted spectral lines of the anode material.